This invention relates to methods of producing an array of MIM type devices, together with address conductors and pad electrodes, on a common substrate for use particularly, although not exclusively, in active matrix addressed display devices, for example liquid crystal display devices.
Known MIM devices, generally comprising a thin film insulating layer sandwiched between two conductive layers across which in use a voltage is applied, can be regared as a kind of diode structure in that they exhibit a non-linear resistive characteristic and have been used in active matrix addressed liquid crystal display devices as switching elements in the addressing of the display device's picture elements. These two terminal devices offer advantages over TFTs also used for such purposes in that they are comparatively simple to fabricate and require fewer address lines, with no cross-overs, on their supporting substrate.
Examples of arrays of MIM devices for liquid crystal display devices and their method of fabrication are described in U.S. Pat. No. 4,413,883 and U.S. Pat. No. 4,683,183.
The MIM addressed display devices consist of first and second glass substrates carrying respectively sets of row and column address conductors with individual picture elements being provided at the region of the intersections of the crossing row and column conductors. A picture element electrode carried on the first substrate is connected electrically to a row conductor via at least one MIM device which is also carried on the first substrate. The MIM devices acts as bidirectional switches having a threshold characteristic and in response to an applied voltage are turned on to allow video data signal voltages to be transferred to the picture elements to cause the desired display response.
Although generally referred to as a Metal-Insulator-Metal device, conductive materials such as indium tin oxide (ITO) can be used as one or both of the "metal" layers and the acronym should be construed accordingly. Moreover, the terms "insulator" and "insulating layer" as used herein are intended to be construed in the wider sense to include semi-insulators and non-stoichiometric materials known in the field of MIM devices. The device switching characteristics are dependent on the composition and thickness of the insulating layer and are determined by the charge transfer mechanisms involved. The switching behaviour of many MIM devices results from tunnelling or hopping of carriers in the thin film insulating layer and in this respect the voltage/resistance characteristic of the device is dependent on the magnitude of the electric field and thus the nature and thickness of the insulating layer. In some forms of MIM devices the mechanism is controlled by the barrier between the metal and the (semi-) insulator. The aforementioned specifications describe various forms of MIM devices using different materials. For the conductive layers suitable materials include nickel, chromium, tantalum, aluminium or other metal. The insulator layer may be of silicon nitride, silicon dioxide, silicon oxynitride, silicon monoxide and zinc oxide may be used for the insulator. Further examples of MIM structures used in display devices, comprising non-stoichiometric materials, are described in EP-A-0182484.
The I-V characteristics of these MIM devices tend to be asymmetric. In order to provide symmetrical characteristics, which readily permit the desirable polarity reversal of addressing signals when driving the display devices, it has been proposed in U.S. Pat. No. 4,413,883 that a pair of MIM devices connected in series back to back configuration or in parallel opposition configuration be used for each picture element, whereby substantially symmetric I-V characteristics are obtained. In a described embodiment, a pair of MIM devices are disposed side by side on a support with respective first terminals of each device being interconnected through, and comprising portions of, a single layer defined on the support. The method of fabricating the pairs of MIM devices together with display element electrodes and addressing conductors involves a number of definition processes requiring a plurality of masks and exposures.
While active matrix substrates of display devices using MIM devices are generally simpler to construct, and thus less expensive, than those using TFTs as switching elements, there is still a need to improve yields and reduce costs further. Also, it has been found in the fabrication of display devices that MIM devices, which are susceptible to the effects of static electricity, are easily damaged as a result of static charges being produced in certain manufacturing operations.